(1) Field of the Invention
The present invention relates to a Venetian blind, particularly a Venetian blind with a rotary tubular headrail.
(2) Description of the Prior Art
Each Venetian blind is composed of a plurality of vertical or horizontal slats running in parallel, tilting of which can adjust indoor darkness and preserve confidence in personal living. It is being extensively used in offices and homes. The conventional horizontal Venetian blind being used can have its slats tilted and lifted, and comprises mainly a rectangular sectioned and non-rotary headrail, a foot rail, two or more ladders each including an outer ladder tape and an inner ladder tape suspending from the headrail to the foot rail to support a plurality of slats with a plurality of connecting tapes located between the outer and inner ladders, a tilter within the headrail and a transmission device therefor to tilt the slats, and a lifter located within the headrail to raise and lower the slats. The tilter and the transmission device are incorporated with a handle to tilt the slats manually, and are substantially in a form of a drum driven by a worm drive through the handle for rotation, include at least to tilt drums each wound by a ladder to support the ladder, and a transmission shaft located between the tilt drums to rotate the tilt drums simultaneously. When the handle is operated, the ladders are moved linearly by the tilter, transmission shaft and the tilt drums, the outer and inner ladders are moved in two opposite directions to tilt the slats between them. The lifter is incorporated with a lift cord, at least two wires extending from the headrail to the foot rail and connected to the lift cord, and a pulley set. When the lift cord is pulled upwards, the pulley set causes the wires tied to the foot rail to displace upward simultaneously to raise the foot rail as well as all or part of the slats, and consequently cause the lower slats to close to the upper slats.
The conventional tilter and lifter for Venetian blind are quite complicated in structure. Their production cost is high, and it is difficult to implement automatic assemble for mass production. In addition, they have the following defects:
(1) The ladders are wound around or connected to the respective ladder drums, hence the distance between the outer ladder tape and the inner ladder tape near the headrail or foot rail will remain fixed and is equal to the diameter of the ladder drum in spite of the degree of tilting, and such a fixed distance will impose an improper limitation to the ability of the upper and lower few slats to tilt. Generally, the maximum degree of the uppermost slat can tilt is about 65 degree. As for tilting of the slats at the intermediate section, their tilting degree is greater than that of the upper and lower few slats for the ladders themselves are soft wires and they are forced to keep close to each other, i.e., the outer ladder tape and the inner ladder tape are closer to each other than the outer ladder tape and the inner ladder tape near the headrail and foot rail. In other words, when the tilter is adjusted to the maximum tilting degree, the few slats at the top and bottom of the conventional Venetian blind can't be fully seal, and such defect would bring inconvenience to person who desires complete confidence or room darkness. Moreover, the difference in tilting degree of the slats would result in an unharmonious sense, and damage the nice appearance of the Venetian blind. PA1 (3) When the slats are tilting, the inner ladder tape and the outer ladder tape are displaced linearly in two opposite directions, and they are subjected to different gravities. One of the ladders are pulled downward by gravity of the slats and the foot rail, while the other is subject to less tension in comparison with that exists when the slats are located horizontally. Such a difference can bring an adverse to each slat's tilting degree. The top end of the ladder subjected to the gravity has the maximum stretch, and them the effect t the tilting degree is the greatest. On the other hand, the tilting degree of the lowest slat is not effected for the ladder there is not stretched. Together with the inconsistent tilting degree described in paragraph (1) above, such inconsistent stretch prevent a seal or near seal from occurring in the last few slats. PA1 (3) Even the slats are maintained horizontal so that both the inner and other ladders are subjected to the same gravity, the stretch at the top section of each ladder is greater than at the bottom section, hence the interval between two consecutive slats are decreasing gradually from the top to the bottom. Such inconsistent intervals would damage the nice appearance of Venetian blind. PA1 (4) The lifter of the conventional Venetian blind uses a pulley and a cord connecting to two wires to pull up the slats and the foot rail. However, in the cause of raising or lowering, the slats and the foot rail may become higher at one end and lower at the other end due to unbalanced force applied to the wires and the ladders, and in the most serious case the slats and the foot rail may be seized and can't be raised or lowered.